One of the earliest forms of engine developed for providing mechanical work was a Rankine cycle engine, often referred to as a ‘steam engine’ because the majority of such engines used steam as their working fluid (and were thus considered to be steam driven). Steam engines were reciprocating engines that typically had a reciprocating piston in a cylinder, with an inlet valve and an exhaust valve (usually at the same end of the cylinder), the piston being connected by a rod and a crank to a flywheel or the like.
During operation of the engine, with the piston at ‘top dead centre’ (referred to as ‘TDC’), the inlet valve was opened, allowing steam to enter from a boiler. The expanding steam drove the piston in its expansion (or power) stroke, whereupon the inlet valve would close, allowing the steam in the cylinder to expand to a lower pressure. As the piston reached ‘bottom dead centre’ (referred to as ‘BDC’), the exhaust valve would open allowing the steam, which was generally still at significant pressure, to escape as the piston travelled back up the cylinder to TDC on its return stroke.
In such an operation, it is ideal to open and close the inlet valve infinitely quickly, and to close the inlet valve early in the power stroke, providing a high expansion ratio. However, in the early 1900's valve actuation technology was limited and poor efficiencies were thus accepted throughout the development of such engines. Indeed, the inability to close the inlet valve early enough was a major factor leading to the development of compound engines (double, triple and even quadruple expansion engines) where steam would be routed to a second, larger capacity cylinder where it was similarly expanded. Sometimes there was also a third, or even a fourth stage where this would be repeated.
While engines of this type generally performed satisfactorily, subsequent developments in engine design produced engines of greater efficiency and higher horsepower to weight ratios, such as the internal combustion engine, the steam turbine and the like. As a result, the use of steam engines fell away, so much so that steam engines became quite rare.
However, with increasing emphasis on environmental and pollution considerations, and with the continuing rise in the price of fossil fuels, there has recently been renewed interest in steam engines, particularly for use in small cogeneration or combined heat and power (CHP) systems.
Accordingly, there is a renewed need for improvements to, in particular, the inlet valve systems for such steam engines and, in general, to the working fluid inlet systems for reciprocating engines of any type where a high pressure gas or vapour is fed to a cylinder in a controlled manner.